The spheroidal graphite cast iron is a casting alloy that develops from 1940s. Since the graphite is spheroidal, its mechanical properties are far better than the gray cast iron, superior to the malleable cast iron and close to the steel. In addition, it has some excellent properties not owned by the steel, e.g., fine castability, machinability, wearability, corrosion resistance and extensive thermal treatment adaptability. Therefore, from 1950s till now, the spheroidal graphite cast iron becomes the most rapidly developing casting alloy: the output of the spheroidal graphite cast iron throughout the world is only 50,000 tons in 1949, while 535,000 tons in 1960, 5 million tons in 1970, 7.6 million tons in 1980, 9.15 million tons in 1990, 13.1 million tons in 2000, and reaches 23 million tons in 2007. The application fields of the spheroidal graphite cast iron are very extensive, and counted in total weight, currently the cast tube and pipe fittings occupy about 40%, the automotive castings occupy about 35%, and other branches (ship, locomotive, diesel engine, agricultural machinery, middle pressure valve, metallurgical machinery, machine tool, hydraulic parts, hoisting and transport machinery, general machinery and nuclear fuel storage and transport apparatus) occupy about 25%. With its excellent properties, the spheroidal graphite cast iron develops rapidly around the world. Countries in the world widely use the spheroidal graphite cast iron to replace the forged steel, cast steel, malleable cast iron and common gray cast iron to manufacture various structural members, so as to save steels and machining working hours, prolong the service life, reduce the usage and maintenance cost, and hence acquire huge economic and social benefits. Presently, the annual output of the spheroidal graphite cast iron all over the world occupies a larger and larger proportion in the total output of ferrous cast metal, and becomes an important index for evaluating the developmental level of the foundry industry of one country.
However, as compared with other metal materials, the spheroidal graphite cast iron has a great “pity”: the properties largely fluctuate during the process of metal solidification and forming, thus the material reliability is decreased, and particularly, for enterprises that produce products such as pipeline and automobile parts on a large scale, an outsize claim may be caused even undetected waste products of millionth occur during the entire production process, then the further development, promotion and application of the spheroidal graphite cast iron is restricted, this is because:
During the production process of the spheroidal graphite cast iron, the mechanical properties of the spheroidal graphite cast iron are mainly determined by the graphite shape formed during the solidification of the liquid iron, instead of the chemical composition such as carbon and silicon, thus the spheroidal graphite cast iron is also called as “magic” metal material having unstable production properties. Since the spheroidization of the spheroidal graphite cast iron is carried out ladle by ladle after the smelting of the liquid base iron, the liquid iron of different ladles may have different properties, and more importantly, the same ladle of liquid spheroidal graphite cast iron after a successful spheroidization will also encounter a degradated spheroidization due to unceasing “resulfurization” and magnesium vapor “escape”, thus the mechanical properties of the castings poured later obviously become poor, and waste castings are produced. This greatly increases the difficulty of quality control during the factory production process, the spheroidal graphite cast iron is also regarded as unstable “low-reliability material”, and its further development is then restricted. Since the liquid spheroidal graphite cast iron will continuously undergo the “degradated spheroidization”, the effective pouring time is very short, and pouring is usually no longer allowed in the factory about ten minutes later when the pheroidization of a ladle of liquid spheroidal graphite cast iron is completed.
Experience shows that when the liquid spheroidal graphite cast iron is heated and insulated in the induction furnace, the electromagnetic stirring and temperature rise of the liquid iron will aggravate the “resulfurization” and magnesium vapor “escape”, and the speed of the “degradated spheroidization” of the liquid iron in the furnace is obviously increased. Thus, the production lines of spheroidal graphite cast iron castings in China scarcely use the pouring electronic furnace, as a result, the phenomenon of temperature drop of the liquid iron is serious during the pouring process, the rejection rate related to the pouring temperature is high, the designed wall thickness of the casting is compelled to be increased artificially, and the material and energy are wasted. Currently in developed countries, the pouring electric furnace provided for the modeling and casting lines of automobile spheroidal graphite cast iron castings is a pressure pouring electric furnace having inert gas (argon, nitrogen) protection. Although such pouring electric furnace solves the problem of unstable pouring temperature, makes the pouring temperature stable and controllable, and prolongs the time at which the degradated spheroidization of the liquid spheroidal graphite cast iron occurs, the degradated spheroidization is not eliminated, and the amount of residual magnesium in the liquid spheroidal graphite cast iron filled into the electric furnace shall be increased to continuously compensate the consumption of nodulizer of the liquid spheroidal graphite cast iron, which not only increases the consumption of nodulizer, but also makes the furnace lining be continuously thickened due to the persistent reaction between the excessive metal magnesium in the liquid iron and the furnace lining, and shortens the maintenance and service cycle of the electric furnace lining. Thus at present, such pouring electric furnace is mainly adopted in the casting production lines where the spheroidal graphite cast iron and the gray cast iron are alternatively used, and is not suitable to enterprises that merely produce the spheroidal graphite cast iron castings. In addition, the air pressure pouring electric furnace requires the seal and automatic pressure adjustment of the furnace gas, thus the device structure is complex, the service and maintenance are difficult, and the material cost is high due to the consumption of argon or nitrogen all the year round.